1. Field of the Invention
The invention is drawn to processes for the catalyzed pyrolysis (or "hydrocracking") of hydrocarbon feedstocks in the presence of hydrogen which use a homogenous catalysis system. The catalysis system consists of a complex of a metal halide Lewis acid, a Bronsted acid and a fluid hydrocarbon dissolved in a second fluid hydrocarbon. The fluid hydrocarbon comprises a C.sub.2 -C.sub.14 alkane.
2. Description of the Prior Art
Hydrocracking refers to a class of processes in which larger molecules are cracked to form smaller molecules in the presence of a catalyst and hydrogen. Hydrocracking processes are particularly useful in the treatment of purified distillate fractions of crude oil.
Crude oils are generally classified according to properties of either the atmospheric distillation tower bottoms (650.degree. F.+) or vacuum distillation tower bottoms (1,050.degree. F.+). For instance, the atmospheric distillation tower bottoms of "light crudes" have less than 5% carbon residue (as defined by destructive distillation in a Conradson Carbon test) and less than 10-15 weight parts per million (wppm) nickel and vanadium. Such feeds can be run directly to a catalytic cracking reactor.
Heavy crudes typically contain 70-85% (vol) of 650.degree. F.+ residue. Such residual oils contain high volumes of carbon residue and are thus regarded as distress stocks by the petroleum industry. The presence of the carbon residue contributes to the production of coke during cracking operations. In addition, high levels of metals in the heavy ends rapidly deactivate the cracking catalyst, leading to uneconomic yields. Thermal processing (such as coking) or, alternatively, deasphalting is therefore normally required to reduce the carbon residues, along with the metals, prior to further upgrading. In addition to being highly contaminated, heavy crudes contain a limited amount of valuable light products such as gasoline and kerosene.
It has long been recognized that solid catalysts, such as AlCl.sub.3 (and more particularly AlCl.sub.3 promoted by HCl), are effective in catalyzing the cracking of a wide variety of hydrocarbons to predominantly lighter hydrocarbons. Catalytic hydrocracking methods are well known and typically are conducted over such solid catalysts at temperatures of 500.degree.-800.degree. F. at pressures of 1000-2000 psi and holding times of 1-3 hours. Two well known types of hydrocracking are the single-stage type and the two-stage type. In the former process, the feedstock is pretreated to remove essentially all sulfur and nitrogen. The hydrocarbon stream is then pyrolyzed in a reactor in the presence of a solid catalyst and hydrogen at a single pass conversion of between 40 and 70 percent. In the two-stage process, a second stage uses the effluent from the single-stage type hydrocracking process (after elimination of the bulk of the sulfur and nitrogen as H.sub.2 S and NH.sub.3, respectively) and a second hydrocracking reactor. The unconverted feedstock is recycled in the second hydrocracking reactor. Because the catalyst in the second hydrocracking reactor operates in an essentially ammonia-free environment, the extent of conversion in this reactor is at a higher level, e.g., 60 to 80 percent per pass.
It is generally recognized that the solid catalyst in such hydrocracking processes reduces the requisite temperature needed for the reaction. In addition, it promotes the hydrogenation,of the cracked products ("hydrocrackate") and dictates the selectivity of cracking.
The hydrocracking processes of the prior art unfortunately render heavy polymers as by-products. These, in turn, produce coke on the solid catalyst. In addition, the processes must be conducted under rather severe operating conditions as noted above. There is an emerging need for improved hydrocracking processes which are not limited by metal buildup and undesirable catalyst coking. Improved hydrocracking processes are further desired which require milder operating conditions and which minimize undesirable low-value process by-products.
Further, there is a tremendous economic interest in developing hydrocracking processes which are suitable for the production of n-paraffins having a specified carbon number, such as, for example, dodecane.
Hydrocracking of high molecular weight paraffin feedstocks, such as hexadecane (cetane), has been extensively studied. Goldfarb et al., (Kinetics and Catalysis, Vol. 22, No. 3, Part 2, pp. 507-513) reported high conversion over a supported NiMo catalyst at 900.degree. F. with a holding time of 40 min. White et al., (Journal of Physical Chemistry, Vol. 68, No. 10, October, 1964, pp. 3085-86) reported 50% conversion at 550.degree. F., 1200 psi, and a holding time of 8 seconds in the presence of a sulfided Ni on silica alumina catalyst. Flinn et al., at Gulf Research (Industrial and Engineering Research, Vol. 52, No. 2, February 1960, pp. 153-156) reported virtually complete reaction with a sulfided 3 wt% Ni catalyst on silica alumina at 702.degree. F., 980 psi and a holding time of about 1 hour.
Unfortunately, the hydrocracking catalysts of the prior art employed with paraffin feedstocks render a hydrocrackate of a wide range of carbon numbers; typically having a preponderance of light ends. For instance, the data from the referenced studies on sulfided Ni/SiO.sub.2 -Al.sub.2 O.sub.3 catalysts demonstrate a broad C.sub.3 -C.sub.13 product distribution at low conversion. Simpler product distribution is obtained at 100% conversion. In the latter, the product is 80 wt% C.sub.5 with the balance being predominantly C.sub.6. The product distribution is consistent with the following reaction network: ##STR1## Two reaction pathways (likely involving two distinct sites) occur on sulfided Ni on SiO.sub.2 -Al.sub.2 O.sub.3 catalyst. Path A (which likely involves a weak acid site) involves breaking the C.sub.16 fragment into two C.sub.8 fragments which, in turn, rapidly crack to yield either C.sub.4 or C.sub.3 and C.sub.5 fractions. Path B (likely involving a stronger acid site) produces C.sub.6 and C.sub.4 fragments. The small fragments do not leave as butane but rather recombine to produce a C.sub.8 fragment which is then fed into the C.sub.8 cracking network. Selectivity between Paths A and B is approximately 50/50.
Hexadecane crackings has also been studied using hydrogenation catalysts such as platinum on silica-alumina. Unlike the acidic catalysts (which produce about 80% C.sub.5) the non-acidic catalysts tend to produce an equimolar mixture of C.sub.4, C.sub.5, C.sub.6 . . . C.sub.12. See, for example, Coonradt and Garwood (Industrial & Engineering Chemistry, Process Design and Development, 1964, No. 3, p. 38). A need exists therefore for a hydrocracking process for use with heavy paraffinic feedstocks which is capable of selectively rendering a desired hydrocrackate of a specified carbon number instead of a range of carbon numbers.